This invention relates to a device or control for preventing overheating of a thermal fusing apparatus utilized in a xerographic reproducing machine for heating and fixing resinous powder images formed on a suitable base or substrate.
Generally, several types of systems for performing such a function as heating resinous powder images are known, among which there are two typical or commonly employed systems. One such system comprises the passing of a substrate between two heating rolls coated with an abhesive material, for example, tetraflourethylene or the like, so as to cause the discreet portions of the resinous powder to coalesce into a tacky state and thereby adhere to the substrate and upon cooling thereof become substantially permanently bonded thereto. The abhesive material performs the function of releasing the tacky material to the substrate.
Another commonly employed system comprises a heating wire which is coiled around a quartz tube wherein a substrate having powder images electrostatically adhered thereto are passed in thermal communication with the coiled wire in such a manner that the heat therefrom causes the resinous powder to coalesce and become tacky.
In the case of the former system, imperfect or no fusing may result unless sufficient pressure is applied between the two rolls which are coated with a non-binding or abhesive agent, such as tetraflourethylene, so that a large pressure or force needs to be applied to the rolls thereby necessitating that the rolls possess a high degree of rigidity to withstand large pressures. Therefore, the heat capacity of such rolls is necessarily increased resulting in a longer time being required for such rolls to be elevated to a temperature sufficiently high to properly fuse the powder images.
In the case of the fuser of the latter type wherein thermal energy is provided by means of a wire coiled around a quartz tube, there is a possibility of fire due to the fact that the wire heating element becomes red hot during fusing. Such a system, therefore, presents problems with respect to operational safety requiring safety devices be provided to prevent the possibility of such dangers.
In an attempt to overcome such problems there has been developed a so-called planar or flat heating type fusing system wherein there is provided a relatively thin planar heating element or unit made of a material with high electrical resistance, for example, nickel-chrome foil. This system is advantageous over the other systems discussed above, in that, it has a greater efficiency because for the same amount of power being supplied a greater surface area is available and faster temperature rise is obtained.
Various types of controls are known for maintaining planar type heating elements at an optimum range of temperatures, such systems are based on the same principle of operation which is the measuring of the operating temperature of the heating element or its environment and the comparing of that temperature with a reference temperature to thereby generate an error or difference signal which is utilized to vary the power supply to the heating element by either causing it to be raised or lowered. Characteristically, such controls require an excess of power for the purpose of accommodating this difference. Since planar heating elements of the type herein contemplated are extremely thin and extremely small in heat capacity, the excess power hereinabove referred to can elevate the temperature of the heating element far in excess of the desired temperature, in a very short period of time, for example, a few seconds.